In suction systems for sucking off liquid being supplied to the system intermittently, there is often need for a valve which opens for a liquid flow while it closes for air suction when the liquid flow ceases. This automatic vacuum interruption on the inlet side of the valve can be desirable from several aspects. One aspect is to minimize the gas intake in the suction installation, and another aspect is to eliminate the suction sound caused by the vacuum. The elimination of a disturbing suction sound is of particular interest within the dental surgery field, and therefore in the following description it is referred to this field in particular, even if the invention is generally applicable within several other technical fields.
At dental surgery installations there is today desirable and in some countries even required through legislation to connect all sewer facilities for e.g. spittoones and suction equipment to an amalgam separation unit before discharge in general sewer systems. Since several dental surgery equipments are often located in the same place at dental treatment-centres comprising several sewer facilities, it is from practical and economic point of view preferable to use a central plant in a separate room, such as a basement room, for vacuum pump, amalgam separator and liquid separator. When such a central plant serves a number of dental surgery units, it is realized that manual on-off control of the vacuum pump is an impossible way to eliminate disturbing suction sound from for example a so called spittoon at a dentist's chair when the liquid supply (rinse water) to the same is shut off. Manually operated valves with an "on"-position for sucking off liquid and an "off"-position for shutting off the suction sound cannot be accepted, since the liquid is so frequently supplied that the valve must be operated very often to prevent liquid overfill, alternatively disturbing suction sounds on the inlet side of the valve, that is in the spitoone.